The purpose of this submission is to provide
the National Transportation Safety Board with US Airways' analysis and
conclusions regarding the circumstances and causes of the crash of USAir
Flight 427. As the investigation is ongoing, US Airways reserves the right
to supplement this submission.
This extensive, three-year investigation has
yielded a great deal of information and analysis. The data demonstrates,
and all parties seem to agree, that USAir Flight 427's rudder moved to a
full-left position shortly after the aircraft encountered wake vortices generated
by a preceding aircraft. It is also clear that the wake vortex encounter
did not directly cause the accident.
The investigation revealed that the Boeing 737
rudder control system has certain anomalies which may have resulted in a
rudder reversal or uncommanded full rudder deflection on the accident aircraft.
As a result, the Board has issued recommendations to correct problems that
might exist in the Boeing 737 rudder control system. In addition, US Airways
has implemented procedures to deal with potential rudder control problems
in the unlikely event they should occur.
This submission does not analyze possible accident
causes that were investigated and found not to be a factor, for they are
quite numerous and undisputed. Similarly, this submission does not analyze
the various possible rudder control system failure modes because the
investigation of this issue is ongoing and modifications of that system already
recommended by the Safety Board should protect against future accidents.
Instead, this submission concentrates on the actions of the flight crew and
facts which establish that the flight crew did not cause the full-left rudder
deflection, and that they acted properly in responding to this emergency
II. EXECUTIVE SUMMARY
A. ACCIDENT SUMMARYOn September 8, 1997,
USAir Flight 427 was approaching the Pittsburgh International Airport at
6,000 feet and 190 knots when it encountered the wake vortices of a preceding
Boeing 727. Approximately three seconds after encountering the wake vortex,
the accident aircraft's rudder suddenly moved to a full-left position. The
aircraft began to yaw and roll left, and the nose began to drop. As the crew
attempted to regain control of the aircraft, the roll and yaw continued,
and the nose continued to drop. Approximately 23 seconds after the onset
of the full rudder deflection, the aircraft impacted the ground in a nearly
vertical attitude, still rolling and yawing.
B. INVESTIGATION AND ANALYSIS1. FLIGHT
CREW QUALIFICATIONSThe flight crew of USAir Plight 427, Captain Peter
Germano and First Officer Charles Emmett, were experienced, highly qualified
and fully trained pilots. They had a combined total of over 14,000 hours
of flight time as USAir pilots, including nearly 8,000 hours in the Boeing
737. Captain Germano had flown with USAir for over 13 years, and First Officer
Emmett for seven years. Each had extensive aviation experience prior to their
USAir employment. USAir flight training records and interviews with other
pilots confirmed that the USAir 427 flight crew were properly trained, capable,
well-respected, and approached their duties with exemplary professionalism.
2. FLIGHT CREW PERFORMANCEThe Aircraft
Performance Group and Human Performance Group studied several aspects of
aircraft and pilot performance in an attempt to determine whether the flight
crew had commanded the full-left rudder deflection that caused the aircraft
to begin its yaw and roll upset and, regardless of the cause of the rudder
deflection, whether the flight crew acted properly in dealing with the emergency.
To make this determination, the Aircraft and Human Performance Groups
investigated several possible indicators of crew actions, including wake
vortex encounters, disorientation and vestibular effects, speech patterns,
rudder pedal damage patterns, B-737 crossover speeds, and unusual attitude
training and procedures.
a. Flight Crew Control Inputs
The facts developed in this investigation do
not support a conclusion that the flight crew commanded the full-left rudder
deflection that caused this accident.
The Aircraft Performance Group conducted flight
tests to determine the reaction of a Boeing 737 when flown into the wake
vortices of a Boeing 727 under the same conditions that prevailed at the
time of the USAir Flight 427 accident. These test encounters resulted in
aircraft attitude deviations almost identical to those experienced by Flight
427 during its wake vortex encounter. The pilots who flew the tests reported
that the wake vortex encounters were easily recoverable and were not
disorienting. They also reported that at no time did they feel they were
close to losing control of the Boeing 737. An airline pilot with extensive
experience in the Boeing 737 participated in the tests and reported that
the test encounters were virtually identical to those he had experienced
while flying the line, and that such encounters are frequent and routine
for airline pilots. The flight tests showed it to be extremely unlikely that
the highlyexperienced USAir Flight 427 flight crew were so startled by a
routine wake vortex encounter that they mistakenly applied and held full-left
rudder and full-right aileron for 23 seconds as the aircraft spiralled into
the ground.The Human Performance Group examined, with the aid of a NASA expert,
the possibility that Captain Germano and First Officer Emmett may have become
disoriented during the wake vortex encounter, leading to an incorrect application
of flight controls. They found that circumstances conducive to vestibular
disorientation -- lack of visual references combined with sudden, violent
aircraft motion or subtle, gradual aircraft motion -- were absent during
the USAir Flight 427 accident. In addition, the cockpit voice recorder tape
revealed the pilots were aware of the aircraft's attitude, but could not
control it. Because the circumstances giving rise to disorientation were
not present, and the pilots were aware of the aircraft's attitude, there
is no reason to suspect that Captain Germano or First Officer Emmett were
disoriented during the accident sequence.The high quality of the Cockpit
Voice Recorder tape made it possible to conduct a detailed analysis of the
voices of Captain Germano and First Officer Emmett during the accident sequence.
The Human Performance Group's analysis, conducted with the assistance of
three experts in the field, showed that First Officer Emmett was flying the
aircraft. The analysis also showed that Captain Gerrnano was not participating
in physical control of the aircraft until immediately prior to impact, if
at all. The voice analysis shows that Captain Germano was attempting to analyze
the nature of the problem while directing the recovery attempt. There is
no evidence that both pilots attempted to simultaneously apply flight control
inputs during the wake vortex encounter or the full rudder deflection that
The cockpit voice recorder also shows that the
pilots' voices during the wake vortex encounter were unexcited. Even several
seconds after the onset of the full rudder deflection, their voices still
did not demonstrate "startle" or panic. The voice analysis studies provide
no evidence that the flight crew were so startled by their wake vortex encounter
that they unknowingly and incorrectly applied full rudder, full opposite
aileron, and held these cross-controlled inputs for 23 seconds as the aircraft
spiralled into the ground.b. Flight Crew Response to Full-Left Rudder
DeflectionAnalysis and flight tests conducted after the accident revealed
that at the time of the wake vortex encounter, USAir Flight 427, although
flying at the correct airspeed, was nonetheless flying at an airspeed below
which the Boeing 737's lateral controls (ailerons and spoilers) were unable
to overcome the roll induced by a fully-deflected rudder. This latter speed
is known as the "crossover speed," although the term and the concept had
not been made known to USAir or the airline industry prior to this accident.
Below the crossover speed, an aircraft that experiences an uncommanded,
fully-deflected rudder cannot be recovered unless the crew accelerates the
aircraft to a speed above the crossover speed, which requires that the crew
immediately descend toward the ground to gain speed as quickly as possible.
Such a maneuver under the circumstances would be completely inconsistent
with training, pilot instincts and expectations, unless the crew had been
aware of the crossover speed concept and the crossover speed for their flight
Post-accident analysis showed That Flight 427's
rudder moved to a fully-deflected position while the aircraft was at or below
the crossover speed. Upon encountering this uncommanded yaw and roll, the
flight crew reacted properly by applying aileron and spoiler opposite the
direction of the uncommanded yaw and roll. The crew, unaware of the crossover
speed concept, let alone the B-737's crossover speed, attempted to maintain
altitude while recovering from the uncommanded yaw and roll. These actions
were proper and consistent with existing knowledge and procedures but,
unbeknownst to the flight crow, quickly placed the aircraft in a position
from which recovery was not possible.C. CONClUSIONSThe data revealed during
this investigation demonstrates that Captain Germano and First Officer Emmett
did not apply full-left rudder during the wake vortex encounter, oppose it
with opposite aileron and spoiler, and hold these cross-controlled positions
for 23 seconds as the aircraft spiralled into the ground. The investigation
did, however, reveal several anomalies the Boeing 737 rudder control system
that may have caused the aircraft's rudder to fully deflect without crew
input or to move opposite to the crew's input.For these reasons, US Airways
concludes that the probable cusee of the accident was an uncommanded, full
rudder deflection or rudder reversal that placed the aircraft in a flight
regime from which recovery was not possible using techniques known at the
time. Contributing to the accident was the failure of the manufacturer to
advise operators of the 737 that there were speeds below which the ailerons
and spoilers could not counteract a hardover rudder.
III. FACTUAL INFORMATION
A. HISTORY OF USAIR FLIGHT 427
On September 8, 1994 at approximately 1903 Eastern
Daylight Time, USAir Flight 427, a regularly scheduled revenue flight from
Chicago, Illinois to Pittsburgh, Pennsylvania, crashed while approaching
the Greater Pittsburgh International Airport.The Captain, Peter Germano,
and the First Officer, Charles B. Emmett, III, were on the last day of a
three-day trip at the time of the accident. The accident flight was their
tenth flight together during the trip. Between them, these pilots had over
40 years of aviation experience and over 21,000 flight hours, approximately
7,700 of which were in the Boeing 737.During the trip, the crew's on-duty
time had never exceeded nine hours during any one day, and off-duty time
had never been less than 13 hours between duty periods. Human Performance
Group Factual Report, Exhibit 14A at 4.Captain William Jackson, a USAir DC-9
Captain, flew in the cockpit observer seat ("jumpseat") during the flight
from Charlotte, North Carolina to Chicago, Illinois, which immediately preceded
the accident flight. Testimony of Captain William Jackson, Transcript of
Proceedings before the National Transportation Safety Board, January 23,
1995, 41 (hereinafter, "Jackson Tr."). Captain Jackson stated that Captain
Germano flew the leg from Charlotte to Chicago. Operations Group Report,
Exhibit 2A at 3. He described the crew as "capable and very professional"
and reported they used aircraft checklists and made all the standard and
required call-outs. Jackson Tr. at 47. Captain Jackson is experienced on
the B737-300, having Hown 2,800 hours in that aircraft as both Captain and
First Officer. Id. at 49.
He did not observe any aircraft problems and
did not see any outstanding Minimum Equipment List notations in the cockpit.
Id. at 45-46.The aircraft, a Boeing 737-300, Registration Number N513AU,
was on its seventh flight of the day when the accident occurred. There had
been no discrepancies reported on any of the previous flights, and there
were no outstanding Minimum Equipment List items. Operations Group Report,
Exhibit 2A at 3. The aircraft's inspections were current, including three
rudder functional inspections required by Airworthiness Directive AD 94-01-07
during the previous six months. Maintenance Records Group Chairman's Factual
Report, October 14, 1994, Exhibit 1 1A at 4.The departure from Chicago and
flight into the Pittsburgh area were uneventful. Air traffic control tapes
and the Cockpit Voice Recorder indicate First Officer Emmett was flying this
leg. Operations Group Report at 4. External and intra-cockpit communications
were routine, including appropriate reading of checklists. Id. The
weather was clear, it was still daylight, and there was a distinct horizon.At
1902 EDT, Pittsburgh Approach Control directed USAir Flight 427 to turn left
to a heading of 100°. As previously directed by Air Traffic Control,
USAir Flight 427 was level at 6,000 feet (MSL) and maintaining 190 knots
indicated airspeed. The landing gear was up with Flaps 1 selected and the
autopilot engaged. The accident aircraft was 4.2 miles behind a Boeing 727
aircraft, Delta Airlines Flight 1083, which was descending to 6,000 feet.
The two aircraft were assigned the same heading.
As USAir Flight 427 approached its assigned heading
of 100° and had almost rolled out of its slight left bank, the flight
data recorder showed small changes in airspeed, attitude, and vertical
acceleration.1 The autopilot was still engaged. Within a second,
the cockpit voice recorder recorded comments by the pilots ("Sheez"; "Zuh")
and a thump on the aircraft. Over the next three seconds, the aircraft rolled
left to approximately 18° of bank, then rolled slightly back to the
right, but never reached a wings level attitude. During this three seconds,
the CVR recorded another thump on the aircraft. The Captain commented, "Whoa,"
and the CVR recorded the sound of the aircraft trim wheel turning at autopilot
trim rates. The FDR recorded an increase in the amount of aft control column
being commanded as the autopilot maintained level flight.At 1903:01 EDT,
the aircraft's heading slewed suddenly and dramatically to the left. The
Captain said, "Hang on," and the CVR recorded the sound of the First Officer
One second after the onset of the sudden yaw,
the Captain said, "Hang on" again as the left yaw continued. The aircraft's
roll attitude, which previously had begun to decrease, suddenly began to
increase to the left and reached 30°. The aircraft's pitch attitude
began to decrease rapidly. At this point, the rate of descent was approximately
2,400 feet per minute ("fpm"). The CVR recorded the sound of the autopilot
being disconnected.One second later, two seconds after the onset of the yaw
event, the Captain again said, "Hang on." The left yaw and roll continued,
and two seconds later (1903:05), the aircraft passed 55° of left bank,
still rolling and yawing. The nose was now 10° below the horizon. The
Captain again said, "Hang on. " The rate of descent at this point was
approximately 3,000 fpm. The control column was moving aft and vertical "G"
At 1903:07, six seconds after the onset of the
yaw, the aircraft's pitch attitude was approaching 20 degrees below the horizon.
The left bank had increased to 70°. The descent rate was now approximately
3,600 fpm. At this point, the aircraft stalled. Left roll and yaw continued,
and the aircraft rolled through inverted flight as the nose reached 90°
down, approximately 3,600 feet above the ground.The aircraft continued to
roll after reaching a vertical dive, but the nose began to rise. Approximately
2,000 feet above the terrain, as the aircraft's attitude passed 40°
nose low and 15° of left bank, the left roll hesitated briefly. At this
point, approximately five seconds prior to impact, the Captain said, "Pull,"
but the aircraft immediately resumed its left roll, and the nose again dropped.
Five seconds later, the aircraft impacted the ground in nearly 80° of
dive, almost 60° of left bank, and at 261 knots indicated airspeed.
The 132 passengers and crewmembers aboard the aircraft were killed.
INVESTIGATION AND ANALYSIS
FLIGHT CREW QUALIFICATIONS
CAPTAIN PETER GERMANO
a. Factual InvestigationAt the time of
the USAir Flight 427 accident, Captain Germano had accumulated 9,112 hours
at USAir. He had accumulated 4,064 hours in the Boeing 737, of which 3,296
were flown as a Captain. Operations Group Factual Report, Exhibit 2A, at
7. Captain Gerrnano's lifetime flight experience in all aircraft was
approximately 12,000 hours. Id.Captain Germano was 45 years old at
the time of the accident. Id. at 5. His flying career began at age
20 in 1969, when he received his Private Pilot Certificate from the FAA.
Id. at 5. He completed United States Air
Force ("USAF") pilot training in December 1973. Human Performance Group
Chairman's Factual Report, Second Addendum, October 5, 1995 at 5. Although
USAF flight records from that period are not available, the syllabus for
USAF pilot training at the time required that each student receive extensive
instruction and demonstrate proficiency in spin recoveries, unusual attitude
recoveries, and acrobatic maneuvers. Human Performance Group Chairman's Factual
Report, Third Addendum, October 27, 1995 at 2.Following USAF pilot training,
Captain Germano flew the 0-2 aircraft, the USAF version of the Cessna 337.
Id. The aircraft was used in the Forward Air Controller role, which
required abrupt maneuvers and rapid changes in aircraft attitude, often at
low altitude. Id. at 2-3. Captain Germano flew the 0-2 from February
1974 to March 1979, logging over 500 hours as Pilot in Command. Human Performance
Group Factual Report, Second Addendum, Exhibit 14X-A, at 5.Captain Germano
received his Commercial Pilot Certificate in 1974. Operations Group Factual
Report at 5. Prior to his employment at USAir, Captain Germano was a flight
engineer for Braniff Airlines. Human Performance Group Factual Report, Second
Addendum, at 5. Captain Gerrnano was hired by USAir in 1981 as a B-727 Second
Officer, then progressed to become a BAC 1-11 First Officer, a B-737 First
Officer, and then a B-737 Captain.
The Operations Group gathered evaluator comments
on Captain Germano's most recent training and evaluation events, as well
as post-accident peer reports. Captain Germano had received five simulator
evaluations and training sessions in the 13 months preceding the accident.
Operations Group Factual Report, Exhibit 2A, at 6. In each of these training
sessions and evaluations, he successfully completed all the required tasks
without a discrepancy. There were no negative comments on Captain Germano's
performance from any of the five Check Airmen who administered these evaluations
and training sessions. Id. The Check Airman who conducted Captain
Germano's requalification simulator in April 1994 reported that the training
went well with no problems. Id. Similarly, the Check Airman who flew
three requalification flights with Captain Germano in May 1994 reported that
Captain Germano was "very meticulous, very professional, paid attention to
detail, ran complete checklists, and followed all procedures." Id. Three
first officers who had flown with Captain Germano during the 60 days preceding
the accident described him as flying "by the book" and as "very proficient,"
"very thorough," and "not excitable." Two of these first officers stated
that Captain Germano's greatest strength was crew coordination. Id.;
Human Factors Group Report, Exhibit 14A, at 3. Captain William Jackson flew
in the cockpit jumpseat on the Charlotte-to-Chicago leg immediately preceding
the accident flight. He reported Captain Germano provided a thorough jump
seat briefing and invited input from the First Officer and from Captain Jackson
on procedures at Chicago, as Captain Germano had not landed there recently.
Human Performance Group Chairman's Factual Report, October 31, 1994, Exhibit
14A, at 5. Captain Germano upgraded to B-737 Captain in September 1988. He
successfully completed each element of the required 12 hours of simulator
upgrade training and 21 supervised line check flights. Captain Germano
successfully accomplished all other training and evaluations received in
the time between his upgrade to Captain and the accident.
Even as far back as his 1988 captain upgrade
training, six years before the accident, Captain Germano's training and
evaluation records indicate he was a very capable pilot.2 Comments
by Check Airmen and First Officers who evaluated or flew with Captain Germano
near the time of the accident consistently painted a picture of a mature,
thorough Captain. Two first officers reported Captain Germano's greatest
strength was crew resource management. This comment is borne out by the testimony
of Captain Jackson, who witnessed Captain Germano's use of Cockpit Resource
Management techniques when he sought the counsel of his fellow pilots before
flying into the complex Chicago air traffic environment.Captain Germano's
flight training record and the testLmony of his fellow pilots indicate he
was a thorough, proficient, and disciplined captain. In addition, Captain
Germano's trading records do not indicate he ever encountered difficulty
with the application of the appropriate rudder at the correct time, even
in the numerous engine-out scenarios practiced and evaluated during his Captain
upgrade training or proficiency and recurrency training simulators. Nor does
the record indicate any difficulty with orientation or controlling aircraft
attitude during the numerous steep turns, approaches to stalls, and other
advanced maneuvers practiced and evaluated in the simulator. To the contrary,
Captain Germano's military training included unusual attitude recoveries,
acrobatics, and spin recoveries. Additionally, for five years he flew a military
mission requiring frequent abrupt changes in aircraft attitude. To a pilot
2The Aviation Investigation Manual
recommends the Operations Group initially examine the pilots' training records
for the preceding two years. NTSB Aviation Investigation Manual,
Vol. II, p. II-F-74. Even though nothing in the pilots' records
for the two years preceding the accident would indicate a need to examine
older records, US Airways' Submission will discuss these pilots' records
as far back as their checkouts in the positions they held at the time of
experience, the routine wake vortex encounter
experienced by USAir Flight 427 would not pose any significant difficulty
in recognition or recovery.
The facts brought to light in this investigation
show clearly that Captain Germano was fully qualified and properly trained
to function as the Captain of USAir Flight 427. His training and experience
indicate he possessed the skill and training needed to recognize, analyze,
and successfully recover a normally functioning aircraft from the routine
wake vortex encounter experienced by USAir Flight 427.
2. FIRST OFFICER CHARLES B. EMMETTa.
First Officer Emmett was 38 years old at the
time of the accident. Human Performance Group Factual Report, Exhibit 14A,
at 3. He started taking flying lessons as a teenager, and began his aviation
career as a corporate pilot. Id. He was hired by Piedmont Airlines
(later merged with USAir) on February 2, 1987 as a First Officer on the F-28
aircraft. At the time of the USAir Flight 427 accident, First Officer Emmett
had accumulated 4,919 hours as a pilot with Piedmont and USAir, all of it
as a First Officer. This time included 3,644 hours in the Boeing 737. Operations
Group Chairman's Report, Exhibit 2A at 9. In all, he had accumulated over
9,000 hours in his flying career Id.
First Officer Emmett received three evaluations
and training sessions in the seventeen months preceding the USAir Flight
427 crash. FIe completed each of these evaluations without a discrepancy.
Id. The Check Airman who administered First Officer Emanett's last
Training session in the simulator on May 12,
1994 recalled that First Officer Emmett was well prepared for the training
and that his performance in the flying and oral evaluations was "sharp. "
A month before the USAir Flight 427 accident,
First Officer Emmett was the "pilot flying" when the B-737 he was flying
experienced a hydraulic system failure necessitating a heavyweight precautionary
landing. Human Performance Group Chairman's Factual Report, October 31, 1994,
Exhibit 14A, at 3. During the incident, First Officer Emmett transferred
aircraft control to the captain, then assisted the captain in executing
diagnostics and accomplishing the successful heavyweight landing. Id.
The captain described First Officer Emmett's performance during the incident
as "great," and indicated First Officer Emmett was calm throughout the situation.
Id.; Operations Group Chairman's Factual Report, Exhibit 2A at 9.
Other Captains who had flown with First Officer Emmett within the 60 days
preceding the accident described his piloting skills as "exceptional" and
his performance as "outstanding." Operations Group Report, Exhibit 2A, at
First Officer Emmett's transition training into
the Boeing 737 began in April 1989. He received four simulator practice sessions,
all elements of which he performed without a discrepancy. As part of the
transition training, he received 12 Initial Operating Experience evaluation
flights in the B-737, all of which he also completed without a discrepancy.
On May 1, 1989, First Officer Emmett successfully completed his B-737 Transition
Proficiency Check, again without a discrepancy. On May 8, 1989, First Officer
Emmett completed four hours of Line Oriented Flight Training (LOFT), which
was graded Satisfactory without comment.
Check Airmen and Captains who flew with First
Officer Emmett within the 60 days prior to the accident praised First Officer
Enunett's flying skills as "exceptional." His flight training record supports
that conclusion. His flying record does not indicate any difficulty with
making appropriate flight control inputs at the correct time. Nor does the
record indicate any difficulty with orientation or controlling aircraft attitude.
Throughout his seven-year career with USAir, First Officer Emmett, like Captain
Germano, had successfully completed numerous training sessions and evaluations
that included single engine maneuvering, steep turns, and other high task
load events with significant potential for disorientation or confusion.
First Officer Emmett was fully trained and qualified
to function as the First Officer aboard USAir Flight 427. His calm performance
during the hydraulic failure incident only a month before the accident further
demonstrates his flying proficiency and knowledge of proper crew coordination
techniques. From his reported "exceptional" flying skills to his recent
performance as a first officer, everything in First Officer Emmett's flying
record indicates he was fully able to recognize, analyze, and successfully
recover a normally functioning aircraft from the routine wake vortex encounter
experienced by USAir Flight 427.B. FLIGHT CREW PERFORMANCEThe investigation
focused on two aspects of the performance of Captain Gerrnano and First Officer
Emmett during their encounter with wake vortex and the subsequent uncommanded
full rudder deflection that caused this accident. The investigation examined
whether the flight crew was the source of the full rudder movement and, whatever
the source of the movement, whether the crew used proper recovery technique
during the ensuing upset.
1. FLIGHT CREW CONTROL INPUTS
The Aircraft Performance Group concentrated its
investigation on reconstructing the flight control inputs made during the
accident sequence. Part of that effort included study of the effect of a
Boeing 727's wake vortex on a Boeing 737. The Human Performance Group's efforts
included study of the pilots' backgrounds and the actions of the pilots in
the final moments of flight. Human Performance Group Chairman's Factual Report,
Second Addendum, October 5, 1995, Exhibit 14X-A, at 2.
One question examined by these Groups was whether
the flight crew commanded full-left rudder in reaction to the wake vortex
then continued to command full-left rudder for 23 seconds while simultaneously
attempting to overcome the rudder deflection with opposite aileron as the
aircraft spiralled into the ground. The Aircraft Performance and Human
Performance Groups investigated several areas in an attempt to determine
if the full rudder deflection that caused this crash was commanded by the
flight crew. Moreover, the Boeing Company provided the Human Performance
Group and the Board with memoranda and a "contribution" intended to support
the theory that Captain Germano and First Officer Emmett were so "startled"
by a routine wake vortex encounter that they input full rudder and held it
until the aircraft impacted the ground. As the following analysis shows,
there is no evidence to support the theory that a pilotcommanded rudder
deflection caused this accident.
a. Wake Vortex Encounteri. Introduction
USAir Flight 427's wake vortex encounter has
relevance to the accident investigation only to the extent that it caused
a momentary rudder input (by the flight crew or through the yaw damper) which,
due to an unknown mechanical malfunction, translated into a hardover or reversed
rudder. The facts of the investigation do not support an inference that the
continuous, full rudder deflection which occurred subsequent to USAir Flight
427's encounter with wake vortex was commanded by the flight crew.
ii. Factual Investigation
Radar data obtained during the investigation
showed that at the time of the onset of the accident event, USAir Flight
427 was approximately 4.2 miles behind a Boeing 727 aircraft, Delta Flight
1083, which was flying approximately the same heading. The descent profiles
of the two aircraft momentarily placed USAir Flight 427 slightly below the
flight path flown by Delta 1083. As the accident event began, USAir Flight
427's FDR recorded slight changes in airspeed, attitude, and vertical "G"
forces, indicating an encounter with the wake vortex of the preceding Boeing
During its investigation of the USAir Flight
427 accident, the NTSB conducted flight tests in which a Boeing 737-300 was
flown into the wake of a preceding Boeing 727 which had been configured with
smoke generators to make the aircraft's wake vortices visible. Group Chairman's
Report of Investigation -- Wake Vortex Flight Test, November 9, 1995, Exhibit
13X-A at 2. Among the purposes of the flights was to determine the aerodynamic
effect of Boeing 727 wake vortices on a Boeing 737. Id.
Three of the pilots who participated in the wake
vortex tests testified before the Board during its public hearings. Lester
Berven is the Supervisory Flight Test Pilot in the Flight Test Branch of
the FAA Aircraft Certification Office in Seattle, Washington. Testimony of
Lester Berven, November 16, 1995 at 1966 (hereinafter, "Berven Tr."). He
holds a Bachelor of Science degree in Aeronautical Engineering and has
accumulated 7,000 total flying hours, of which approximately 3,500 are in
certification or engineering flight test. Id. at 1967. He is rated
in all Boeing aircraft except the B-707. Id. In his capacity as an
FAA supervisory test pilot, he flies certification flights and supervises
the activities of five other pilots. Id. He participated in the
certification of the B-737-300 aircraft in 1984. Id. at 1968.
Michael Carriker is the Senior Engineering Project
Pilot for the B-737 at the Boeing Company. Testimony of Michael Carriker,
November 16, 1995 at 2083 (hereinafter, "Carriker Tr."). In that capacity,
he contributes to new aircraft projects and continuing improvements. Id.
at 2084. He holds a Bachelor of Science degree in Science and Aeronautical
Engineering, was a Navy test pilot, and is rated in all current Boeing production
aircraft. Id. He has amassed approximately 5,000 hours flight time.
Captain John Cox is a B-737 captain with USAir,
and the Chairman of the Airline Pilots' Association Central Safety Conunittee
at USAir. Testimony of Captain John M. Cox, November 16, 1995 at 2181
(hereinafter, "Cox Tr."). He has flown professionally for approximately 22
years. Id. at 2152. He has accumulated a total of 12,000 flight hours,
of which approximately 8,000 have been in the B-737.
Each of these pilots described the flight test
wake vortex encounters. Mr. Carriker testified that when encountering the
B-727's wake vortex, the B-737 would react to the vortex
in the roll axis. Carriker Tr. at 2106. The
encounters did not result in a large or sustained yaw or a large heading
change. Id. at 2109; Berven Tr. at 2007. Each encounter resulted in
a vertical G spike, followed by a rolling moment. Carriker Tr. at 2107. At
no tune did Mr. Carriker feel he was losing control of the aircraft during
these wake vortex encounters. Id. at 2110.Mr. Berven participated
in the wake vortex test encounters conducted at four and three miles behind
the B-727. Berven Tr. at 2003. He noted that the encounters with the vortex
typically resulted in a 10~ to 20° excursion in bank angle when the
aircraft was being handflown or flown on the autopilot during the encounter.
Id. at 1999. The maximum bank angle Mr. Berven experienced was 30°,
and that was during a wake vortex encounter flown without any pilot or autopilot
input to counter the vortex effects. Id. Mr. Berven found the autopilot
did an effective job of controlling the aircraft during the wake vortex
encounters. Id. at 2010. In encounters with the autopilot on, the
aircraft typically did not roll more than ten degrees. Id. During
none of the wake vortex encounters did Mr. Berven feel that the aircraft
was out of control or even on the verge of being out of control. Id. at
Both Mr. Berven and Captain Cox stated that staying
in the vortex was difficult because the dynamics of the aircraft and vortex
resulted in the aircraft being Spit out" of the vortex rather rapidly. Berven
Tr. at 2000; Cox Tr. at 2165. All three pilots noted that the average duration
of an encounter with a wake vortex during the test was on the order of two
seconds. Berven Tr. at 2000; Cox Tr. at 2165; Carriker Tr. at 2110. Mr. Carriker
testified that it was possible to stay in the effect for up to three or four
seconds, but only if the intent was to do so.
Carriker Tr. at 2110. Because the two wake vortices
rotated in opposite directions, transitioning from one vortex to another
during an encounter tended to correct the initial roll upset. Id.
Mr. Berven and Captain Cox also noted that wake
vortex encounters are common in line flying. Berven Tr. at 2005; Cox Tr.
at 2164. Captain Cox stated that wake vortex encounters occur on the order
of two or three times during a three or four day trip. Cox Tr at 2185. Captain
Cox noted that the encounters he witnessed during the tests were representative
of the wake vortex encounters he has experienced while flying the line. Cox
Tr. at 2184. In both the tests and line flying, the maximum bank angle he
typically has seen is 20 to 25 degrees. Cox Tr. at 2165-66. Captain Cox stated
that wake vortex encounters during the test and during line flying are "not
that disruptive a condition." Id. at 2164.
During the wake vortex encounters, the yaw damper
of the B-737 was providing almost continuous input to the rudder, so much
so that its input interfered with obtaining the desired data, leading the
engineers to request that the pilots turn the yaw damper off during the
encounters. Carriker Tr. at 2107. Id.
The radar, flight data recorder, and cockpit
voice recorder data show that USAir Flight 427 almost certainly encountered
the wake vortex of the preceding B-727. The radar data places USAir Flight
427 behind and slightly below the flight path of the B-727 at the
time of the onset of the accident sequence. The FDR recorded excursions in
roll, airspeed, and vertical "G" without an accompanying pitch attitude change.
The CVR recorded thumps on the aircraft which later testing showed were
consistent with the impact of a wake vortex on the fuselage of a B-737. However,
the wake vortex encounter was not the cause of the accident.
The pilots who participated in the NTSB's wake
vortex testing described the roil rates and magnitudes they encountered,
and what they described was almost identical to the roll rates and magnitudes
recorded on the USAir Flight 427 Flight Data Recorder. During the test,
encounters with the wake vortex at approximately 4 miles behind the generating
aircraft resulted in roll axis upsets of less than 25° when the autopilot
was engaged, as it was on USAir Flight 427. Like USAir Flight 427's encounter
with a wake vortex, these test encounters did not generate significant yawing
moment.The USAir Flight 427 FDR data shows that the aerodynamic reaction
of the accident aircraft to its wake vortex encounter was identical to the
routine wake vortex encounters described by Captain Cox and Mr. Berven. The
maximum bank angle after the wake vortex encounter began, and before the
onset of the uncommanded yaw event, was approximately 18°. Group Chairman's
Report of Investigation, Flight Data Recorder Factual Report, November 28,
1994, Exhibit 10A, Attachment IV. Even the most extreme estimates of the
roll rate the aircraft encountered during the wake vortex encounter was
approximately 11° per second, but the aircraft rolled less than 10°
at that rate. See, Boeing Contribution to the USAir Flight 427 Accident
Investigation Board, September 25, 1996, at 4-6. Until the onset of the full-left
rudder, the maximum yaw rate during the wake vortex encounter was negligible.
Id. Until the onset of the full-left rudder, bank angle changes during
the wake vortex encounter, though rapid, were slight. The F1:)R data clearly
show the USAir Flight 427 wake vortex encounter was nothing out of the ordinary.
It was virtually identical to the types of wake vortex encounters that are
common in line flying.
Such wake vortex encounters are not unusual in
line flying, and any initial surprise experienced by the pilots on encountering
a wake vortex quickly changes to recognition, analysis, and recovery. Cox
Tr. at 2170. Captain Cox, the one pilot with extensive line flying experience
who flew these tests, stated the vortex encounters are just not that disruptive.
The crew of USAir Flight 427 had a combined total of nearly 8,000 hours of
Boeing 737 time, and over 14,000 hours as airline pilots for USAir. Captain
Germano had flown the line at USAir for over 13 years, and First Officer
Emmett had flown the line for seven years. Each had additional aviation
experience prior to their USAir employment. A wake vortex encounter would
have been routine for such highly-experienced crew members.Listening to the
pilots' reactions on USAir Flight 427's Cockpit Voice Recorder tape also
reflects that these pilots were not so startled by this routine wake vortex
encounter that they applied incorrect flight control inputs and held them.
After the pilots' initial reaction ("Sheez," "Zuh"), the only comment either
pilot made before the full-left rudder began is the Captain's comment, "Whoa.
" His voice on the CVR is neither startled nor panicked; rather, the statement
is nothing more than commentary. As the event progressed from a wake vortex
encounter to an uncommanded rudder deflection or reversal, the voices of
the pilots understandably express rising concern over the performance of
the aircraft. However, an expletive spoken by First Officer Emmett some four
seconds after the onset of the unconunanded rudder deflection is made in
a calm, but concerned, tone. The evidence shows that these pilots were not
so startled by this routine wake vortex encounter that they incorrectly applied
full-left rudder, countered it with full-right aileron, then held these full
cross-controlled positions for 23 seconds while spiralling toward the ground.
Boeing's February 28, 1996 memorandum to the
Chairman of the Human Performance Group invites a chain of inferences from
44 anecdotal aircraft incident reports concerning wake vortex encounters.
The first inference invited by this memorandum is that flight crews are commonly
so "startled" by encounters with wake vortex that they input improper flight
controls or put in proper flight controls but forget to take them out. Most
of the encounters cited were behind "heavy" category aircraft, or at short
range, or at low altitude, or under some combination of these circumstances.
It should also be noted that in the 44 reports of wake vortex encounters
cited in the Boeing memorandum, only two of the events even arguably included
an incorrect application of flight controls by a crew member. Neither of
these two incident reports indicate the crew held the incorrect input for
more than a few seconds.
Relying on the same data, Boeing's September
25, 1996 "contribution" to the Board concludes that the crew of USAir Flight
427 were "startled" by their wake vortex encounter, leading to the incorrect
application of full-left rudder and full-right aileron until the aircraft
spiralled into the ground 23 seconds later. In support, the "contribution"
breaks the movements of USAir Flight 427 during the wake vortex encounter
into tenth-of-a-second increments and suggests that the roll accelerations
experienced were so far above what the crew would normally experience that
they could not cope. However, the bank angle changes that occurred at these
rates were sometimes as small as two or three degrees and never more than
10 degrees. They were, in essence, momentary jolts rather than dramatic,
USAir Flight 427 experienced a routine wake vortex
encounter that resulted in several rapid, small, momentary roll angle changes.
The accident aircraft's roll rates and accelerations were similar to those
documented during the NTSB's wake vortex testing. See, Group
Chairman's Report of Investigation, Wake Vortex
Flight Test, Exhibit 13X-A, November 9, 1995 at 27A - 34B. USAir Flight 427's
encounter with wake vortex resulted in the type of "bounce" or "light to
moderate turbulence'' described by the pilots who flew the wake vortex test.
Carriker Tr. at 2110; Berven Tr. at 2007. Such encounters are quite common
in line flying. Cox Tr. at 2185. Given Captain Germano's and First Officer
Emmett's experience, these relatively benign motions resulting from a routine
wake vortex encounter would not have led to an extreme misapplication of
The ultimate inference the Boeing contribution
would have the Board draw is that the crew of USAir Flight 427 applied full-left
rudder during the wake vortex encounter and held it there while applying
fully cross-controlled ailerons until impact some 23 seconds later. However,
the facts revealed during this investigation simply do not support an inference
that the crew of USAir Flight 427 reacted to the wake vortex encounter by
incorrectly applying flight control inputs. Wake vortex encounters are common
events to line pilots and are relatively benign, seldom resulting in more
of an upset than 25° of bank, as was the case with USAir Flight 427.
Line pilots routinely deal with recoveries from such encounters. Few, if
any, of the reported wake vortex encounters cited to support the inference
indicate an incorrect application of flight controls, inadvertent or otherwise.
Similarly, none indicate any instance in which correct controls were applied
then inadvertently held after they were no longer needed. Whatever the cause
of the rudder deflection that led to the crash of USAir Flight 427, there
is no evidence to suggest it was an incorrect or inadvertent pilot input
brought on because the crew reacted incorrectly to a wake vortex encounter.
B. Disorientation and Vestibular Effectsi.
IntroductionThe Human Performance Group investigated the possibility
that the crew of USAir Flight 427 may have experienced spatial disorientation
during the wake vortex encounter, causing them to misapply the flight controls.
The evidence discovered by the investigation shows the pilots were not
disoriented.ii. Factual InvestigationSpatial disorientation is usually
associated with degraded out-of-cockpit vision, coupled with changes in aircraft
accelerations that are either sudden and violent or subtle and gradual. Letter
from Malcolm M. Cohen, Ph.D. to Malcolm Brenner, Ph.D., September 21, 1995,
at 1 (hereinafter, "Cohen Report"). The Human Performance Group asked Malcolm
M. Cohen, Ph.D., NASA Ames Research Center, to examine relevant information
from the accident investigation in an attempt to determine if disorientation
or vestibular effects could have played a role in the accident. Human Performance
Group Chairman's Factual Report, Second Addendum, October 5, 1995, Exhibit
14X-A, at 6. Dr. Cohen underwent repeated simulations of the USAir Flight
427 event in the NASA Vertical Motion Simulator, using large physical motions
to produce a high fidelity reconstruction of the event. Id.Dr. Cohen
concluded that the accident scenario did not contain evidence of the factors
normally associated with disorientation. Cohen Report at 1. The accident
occurred during daylight hours in clear air with good visibility and a clearly
defined horizon. Human Performance Group Factual Report, Exhibit 14X-A, at
6. The motion of the aircraft after the onset of the accident event did not
exhibit the types of accelerations that would be conducive to
disorientation. Cohen Report at 2. The motions
of the aircraft as the event progressed were relatively gradual and nearly
continuous. Id. The pilots' comments indicated they were fully aware
of their trajectory, but they were unable to change it. Id. This evidence
is consistent with the testimony of two pilots who participated in the wake
vortex testing, who stated that the wake vortex encounter is not a disorienting
event. Cox Tr. at 2170-71; Carriker Tr. at 2111.iii. AnalysisDr. Cohen's
analysis indicates the crew of USAir Flight 427 were not disoriented by the
wake vortex encounter that marked the beginning of the accident sequence.
The weather was clear and the horizon distinct. The change in aircraft attitude
happened gradually and continuously, and the pilots' comments indicate they
were aware of the change but could do nothing about it. This simply was not
an event that involved the sudden placement of an aircraft in an extreme
attitude, nor was it a case where subtle changes in aircraft attitude went
unnoticed by the crew until an extreme attitude had developed. Pilots who
participated in the NTSB's wake vortex testing testified that encountering
a wake vortex is not a disorienting experience, and the facts developed during
the investigation indicate the pilots were aware of the attitude of the aircraft
at all times during the accident sequence. Therefore, disorientation was
not a factor in causing this accident.
c. Speech Patternsi. IntroductionThe Human
Performance Group investigated the speech patterns of the USAir Flight 427
flight crew captured on the Cockpit Voice Recorder. The investigation revealed,
not surprisingly, that the pilots exhibited rising stress as the accident
event progressed. The analysis also showed excellent crew coordination before
and during the event and that First Officer Emmett was physically flying
the aircraft.ii. Factual InvestigationThe Human Performance Group
studied the speech patterns of the pilots using the Cockpit Voice Recorder
tape. Three consultants provided input to the Group: an exercise physiologist
who focused on breathing patterns and physical exertion; a NASA Human Performance
Researcher who focused on intra-cockpit communication; and a Russian acoustics
analyst who focused on psychological stress and physical effort. Speech
Examination Factual Report, May 5, 1997 at 5.There was no audible grunting
or straining indicative of physical exertion heard from Captain Germano before
or immediately after the onset of the accident sequence. Letter from Scott
Meyer, Ph.D. to Malcolm Brenner, Ph.D., March 29, 1996, paragraph 7 (hereinafter,
"Meyer Report"). Captain Germano's speech patterns indicate he did not exhibit
signs of physical exertion until about 4.8 seconds prior to the aircraft's
impact with the ground. This straining may have been as a result of "G" forces
on the aircraft or of his participation in manipulating the flight controls.
Letter from Alfred S. Belan to Malcolm Brenner, Ph.D., March 26, 1997, at
3 (hereinafter, "Belan Report").
Shortly after the onset of the accident sequence
First Officer Emmett made several rapid grunting exhalations. Meyer Report
at ~ 5; Belan Report at 4. In addition, analysis of the USAir Flight 427
Cockpit Voice Recorder tape indicated seven occasions after the accident
sequence began in which First Officer Emmett keyed the yoke-mounted radio
microphone switch. Id. at 4. Human Performance Group Chairman's Factual
Report, Second Addendum, October 5, 1995, Exhibit 14X-A, at 4. During none
of these times did First Officer Emmett direct conversation outside the aircraft.
Conversely, Captain Germano keyed his microphone switch only one time during
the accident sequence, and that was to make an emergency transmission directed
to the air traffic control agency. Id.Analysis of the intra-cockpit
communications indicated there was no reluctance of the crew to seek and
incorporate information from each other, and the level of coordination and
communication was appropriate. Letter from Barbara G. Kanlci, Ph.D. to Malcolm
Brenner, Ph.D., December 2, 1996 at 3. During the accident sequence, the
Captain's comments ("Hang on"; "What the hell is this?"; "Pull") were mainly
limited to commands and attempts to evaluate the situation. Belan Report
at 3.iii. AnalysisAnalysis of the CVR and air traffic control tapes
shows that First Officer Emmett was the pilot flying the aircraft up to the
time of the wake vortex encounter. After the encounter, there was no indication
on the CVR of a change of aircraft control. First Officer Emmett's speech
patterns at the beginning of the accident sequence showed forcible exhalations
indicating physical exertion. This evidence suggests First Officer Emmett
was straining while manipulating the aircraft's controls. Meyer Report at
2. Conversely, the lack of such straining in Captain
Germano's speech patterns suggests Captain Gennano
was not manipulating the controls. Further, First Officer Emmett keyed his
yoke-mounted microphone switch seven times during the accident sequence without
making a deliberate transmission. Such inadvertent microphone keying can
be an indicator that a pilot is manipulating the control wheel. Human Perfonnance
Group Factual Report, Second Addendum, Exhibit 14X-A at 3. Conversely, Captain
Germano keyed his microphone switch but once, and that was done in an attempt
to notify air traffic control of USAir Flight 427's emergency. Id. at
4. These facts strongly infer that First Officer Emmett was manipulating
the flight controls during the wake vortex encounter and the subsequent flight
control malfunction and that Captain Germano was not.Analysis of the CVR
also shows the crew used good crew coordination throughout the accident sequence.
In fact, analysis of the entire 30-minute CVR tape showed good cockpit resource
management throughout the last half hour of the flight. During the time before
the onset of the accident sequence, each pilot sought and incorporated
information from the other, and the level of communication and coordination
was appropriate for the task. Captain Germano's statements during the accident
sequence were in the nature of commands and attempts to evaluate the situation
and were proper in that context.
The Boeing February 28, 1996 memorandum to the
Chairman of the Human Performance Group and the September 25, 1996 "contribution"
to the Board suggest that the crew was startled by the wake vortex encounter,
perhaps leading to both crew members manipulating the controls during the
wake vortex encounter and subsequent flight control malfunction. The inference
is that the pilots may have been making contrary control inputs. The inference
is not supported by the evidence, however. The evidence indicates that the
pilots were not startled by this
routine wake vortex encounter and reacted properly
to it. In addition, the speech analysis evidence shows that First Officer
Emmett was in physical control of the aircraft during both the wake vortex
encounter and the subsequent attempt to recover from the uncommanded rudder
deflection. The evidence is also clear that while First Officer Emmett flew
the aircraft, Captain Germano properly provided direction and attempted to
analyze the situation. This evidence indicates the pilots performed their
proper duties throughout the accident sequence, and there was no confusion
about which pilot was manipulating the flight controls.
d. Rudder Pedal Damage Patternsi.
After admitting there was insufficient pathological
information on which to base an opinion, the Deputy Medical Examiner for
the Armed Forces Institute of Pathology provided an opinion to the Human
Performance Group Chairman concerning the forces being applied to the rudder
pedals by the pilots at impact. Because the metallurgical evidence currently
available is at best ambiguous, and the pathology expert was not qualified
to render an opinion on this ambiguous metallurgical data, no conclusions
can or should be drawn concerning the forces being applied to the rudder
pedals at impact. In fact, to the extent a conclusion can be down from the
metallurgical data, it is that a substantial amount of force was applied
to all four rudder pedals and/or their mounting structures at some
point during the impact sequence.
ii. Factual Investigation
Metallurgical examination of the pilots' rudder
pedals and mounting hardware showed that all four of the pilots' rudder pedals
bent forward during the impact sequence. Both pilots' right rudder pedals
bent forward 20° and remained attached to their mounting pivot lugs.
Captain Germano's left rudder pedal sheared from
its mounting pivot lug after also bending forward 20°. First Officer
Emmett's left rudder pedal sheared from its mounting pivot lug after the
pedal had bent forward 5°. System Group Chairman's Factual Report of
Investigation, December 21, 1994, Exhibit 9A at 40; Metallurgist's Factual
Report No. 95-43, December 27, 1994, Exhibit 9B, at 1. The NTSB's metallurgical
analysis did not make any findings as to pressure being applied to the pedals
Dr. David W. Hause, Deputy Medical Examiner for
the Anned Forces Institute of Pathology, reported that while determination
of rudder pedal position can be inferred from the study of the pilots' remains,
the extent of body disruption, quantity of recovered remains, and incomplete
reassociation of the remains in this case made an analysis based on forensic
pathology principles impossible. Letter from David W. Hause to Malcolm Brenner,
Ph.D., January 22, 1996, Attachment 8 to Human Performance Group Chairman's
Factual Report of Investigation, Fourth Addendum.
Notwithstanding this lack of information, Dr.
Hause went on to provide an opinion to the Human Performance Group in which
he "infer[red] the possibility" that both pilots were "symmetrically applying
pressure to their respective left rudder pedals at the time of ground impact."
Id. He based his opinion on the NTSB's metallurgical analysis of the
pedals and attaching hardware. Id. Dr. Hause claimed he was able to
"infer the possibility" that at impact both pilots were "symmetrically applying"
"strong pressure" to the pedals with the "left knee locked" and "the majority
of body weight concentrated on the left foot. " Id.
There is no reason, based on the investigation
record, to question the expertise of Dr. Hause as a pathologist. Indeed,
Dr. Hause properly rendered an expert opinion in the field of pathology when
he found there was insufficient medical evidence on which to determine rudder
pressure being applied at the time of impact. Dr. Hause is not, however,
an expert in metallurgy and is patently unqualified to render an expert opinion
based upon metallurgical evidence. Since Dr. Hause based his inferences on
data outside his field of expertise, his conclusions should be disregarded.
Furthermore, the shear patterns of the rudder
pedals on the accident aircraft are at best inconclusive as to the pressure
being applied to them at the time of impact or at any other point during
the impact sequence. The fracture patterns of the rudder pedal attachment
hardware from USAir Flight 427 do not suggest an application of force by
the pilots to any of the rudder pedals. Captain Germano's left and right
pedals bent forward an equal amount (20°), but the left one sheared
off while the right one did not. There is nothing in this pattern to suggest
more pressure on one pedal than the other. First Officer Emmett's right pedal
also bent forward 20° and did not shear off, while his left pedal bent
forward less than 5° before shearing. No metallurgist has provided an
opinion as to the significance of these bending and shearing patterns, but
they would not seem to indicate that one pedal was under greater pressure
than the other at impact, although they may provide some indication as to
the relative strength and flexibility of the attachment hardware. In fact,
to the extent a conclusion can be drawn from the metallurgical data, it is
that a substantial amount of force was applied to all four rudder pedals
and/or their mounting structures at some point during the impact sequence.
It is also significant that the aircraft impacted
the ground nose first at 261 knots, approximately 80° nose down, in
60° of left bank, and in a significant sideslip. A great deal of aircraft
structure undoubtedly impacted the rudder pedals with enormous force as the
aircraft telescoped on impact. Given the left bank and sideslip, it is also
probable that the force vectors would focus more on the left side of the
aircraft.Dr. Hause's inferences are nothing more than unfounded speculation
by an unqualified witness based on ambiguous and inconclusive data. Because
the metallurgical analysis currently available is at best ambiguous, and
Dr. Hause was not qualified to render an opinion on this data, no conclusions
can or should be drawn concerning He forces being applied to the rudder pedals
at impact.2. FLIGHT CREW RESPONSE TO FULL-LEFT RUDDER DEFLECTION
This section addresses the flight crew's response
to the full-left rudder deflection experienced by the accident aircraft.
Because the aircraft was at or near its "crossover speed" (a speed which
was not communicated to USAir until after this accident), and there was no
known reason for the flight crew to maintain or increase airspeed by descending,
the aircraft quickly departed from controlled flight notwithstanding proper
efforts by the flight crew to maintain control. While unusual attitude training
is useful and appropriate for airline flight crews, it would not have affected
the outcome of this accident given the full-left rudder deflection and the
accident aircraft's crossover speed.
a. Crossover Speedi. Introduction
Post-accident flight tests conducted in a Boeing
737-300 aircraft revealed that 190 knots indicated airspeed ("KIAS") was
at or very near the "crossover speed" for the weight and configuration of
USAir Flight 427. Below that speed, ailerons and spoilers are insufficient
to stop the roll induced by a full rudder deflection. Termed the "crossover
speed, " this information was not provided to USAir or the airline industry
prior to this accident.When the full-left rudder movement occurred, USAir
427's flight crew applied lateral controls to counteract the roll and increased
aft yoke pressure to maintain altitude while they analyzed and corrected
the problem. Although this was proper technique, these actions quickly placed
the aircraft in a position from which recovery was impossible. Unknown to
the flight crew and the industry, the aircraft's crossover speed required
an increase in airspeed, and a corresponding loss of altitude, to accomplish
ii. Factual InvestigationAt the beginning
of the accident sequence, USAir Flight 427 was in level flight at 190 KIAS,
as directed by Air Traffic Control. Specialist's Factual Report of Investigation,
Cockpit Voice Recorder, October 5, 1994, Exhibit 12A at 24. Upon acknowledging
this speed, the crew of USAir Flight 427 selected Flaps 1. Id. This
configuration was proper according to both the manufacturer's and USAir's
maneuvering speed schedules.Early in the investigation, the Aircraft Performance
Group conducted tests in the Boeing Multipurpose Engineering Cab ("MCAB")
simulator. These tests indicated that the B-737's
ailerons and spoilers provided lateral control
authority sufficient to counteract a fully-deflected rudder and maintain
control of the aircraft.On October 20, 1994, the FAA began a Critical Design
Review of the Boeing 737 flight control system, with emphasis on the lateral
and directional flight controls. Boeing 737 Flight Control System
Critical Design Review Report, May 3, 1995 at 1. The CDR team concluded that
a number of possible failure modes existed in the B-737 which could result
in loss of rudder control and subsequent uncommanded, sustained, full rudder
deflection or reversal. Id. at 16. The CDR team concluded that because
of this potential, lateral flight controls must be "fully available and powerful
enough to rapidly counter the rudder and prevent entrance into a hazardous
flight condition. " Id.During its study, the CDR group conducted tests
in the Boeing MCAB sunulator. Id. at 11. These tests provided further
data on the aircraft's controllability with a full, sustained rudder deflection,
including rudder hardovers. Id at 12. The group found that the lateral control
system could overcome the roll induced by a hardover rudder, except at the
i90 KIAS/Flaps 1 data point. Id. At this point, recovery was possible
but was very slow and required prompt, precise pilot control of pitch and
airspeed to preclude entering an inverted attitude. Id. For example,
one of the simulator recoveries from a rudder hardover at the 190 KIAS/Flaps
1 configuration required two separate descents to gain airspeed, and 35 seconds
passed before the simulator was brought under control. Id. at A-19.
The report stated that during this exercise, "recovery from yaw was in doubt."
Id. The amount of altitude lost was not reported.
During September and October 1995, the NTSB,
with the participation of USAir, Boeing, ALPA, the FAA, and NASA, conducted
flight tests using a Boeing 737-300 aircraft provided by USAir. One purpose
of these flight tests was to verify the accuracy of the Boeing MCAB simulator's
B-737 flight parameters. Berven Tr. at 1973. One data point during the flight
test was designed to determine the aileron and spoiler deflection required
to counteract the roll caused by a full rudder deflection at various airspeeds
and flap settings. Id. at 1972. The flight tests and later engineering
analyses revealed that for a B-737 aircraft in the same configuration as
USAir Flight 427, there was a "crossover speed" near 190 knots. ''Crossover
Speed" is an engineering term that refers to the speed at which one set of
flight controls exactly offsets the effects of another set of flight controls,
in this case ailerons and spoilers versus the rudder. Above or below that
speed, one set of flight controls is predominant. Id. at 1980. During
the test flights, it was discovered that at speeds near 190 knots in level,
steady-heading side slips, lateral controls (ailerons and spoilers) were
sometimes insufficient to stop aircraft roll induced by a full rudder deflection.
Cox TR at 2161. At the time of the USAir 427 accident, airline pilots were
not trained in or aware of the concept of "crossover speeds." Cox Tr. at
2191-2. Neither the term nor the concept appeared in materials the manufacturer
provided to the airline industry.
The flight tests also revealed that rudder travel
in the B-737 was greater than that programmed into the Boeing MCAB simulator;
that is, the aircraft was shown to possess more rudder authority than was
programmed into the simulator for the 190 KIAS/Flaps 1 data point. Because
of these findings, the MCAB simulator was modified to more accurately reflect
the rudder authority actually available in the aircraft.
After these modifications, Mr. Berven attempted
recoveries in Boeing's MCAB simulator from full dynamic rudder deflections.
Id. at 2021. After the rudder deflection, Mr. Berven delayed three
seconds to simulate a pilot's recognition time, then started a recovery.
If he disregarded altitude loss and allowed airspeed to increase, the roll
typically could be reversed at a 75° bank angle. When he attempted to
keep the aircraft level and maintain 1gO knots, the aircraft could not be
returned to wings level flight -it continued to spiral in a 70° bank.
Id. at 2022. If he allowed the airspeed to accelerate to above 200
knots, however, the aircraft would begin to recover. Id.
Neither Mr. Berven nor Captain Cox believed a
typical airline flight crew would have attempted to deal with a rudder hardover
-- assuming they knew such a malfunction could occur and had a way to recognize
it -- by descending to increase airspeed and improve aileron/spoiler
effectiveness. Id. at 2036; Cox Tr. at 2190. Instead, when a pilot
is faced with a flight control malfunction, he or she would instinctively
preserve altitude while attempting to resolve the problem. Berven Tr. at
2037. In fact, prevailing unusual attitude recovery techniques call for aft
yoke pressure when recovering from a nose low attitude with less than 60-90C
of bank.In December 1995, USAir increased by 10 knots its Boeing 737 minimum
maneuvering airspeeds for the flaps 1, 5, and 10 settings at gross weights
at and below 117,000 pounds. Boeing stated it had "no technical objection"
to these changes, which cleared the way for USAir to implement the new speeds.
Some, but not all operators have followed USAir's action. Boeing has not
affirmatively recommended the changes, stating that the increase does not
provide significant technical benefits to directional control.
USAir also worked to develop a procedure for
handling a hardover rudder. Among other things, the procedure calls for the
flight crew to maintain an airspeed at or above the new maneuvering speed,
even if altitude is lost in the process. That procedure was ultimately
incorporated into an FAA Airworthiness Directive and is now a part of the
Boeing 737 Pilot's Handbook.
In June 1997, Boeing provided the NTSB with charts
purporting to depict the effect of bank angle on crossover speeds. This data
was not developed as part of the NTSB investigation and the underlying data
and formulae were not provided to the NTSB. While the charts contain certain
anomalies, they do indicate that B-737 crossover speeds increase with bank
angle or, more correctly, with angle of attack or G loading.
At the time the accident sequence began, USAir
Flight 427 was cruising at an assigned altitude and airspeed of 6,000 feet
and 190 KIAS. Commensurate with the manufacturer's flap maneuvering speed
schedule and USAir's B-731-300/400 Pilot's Handbook, the crew had configured
the aircraft with Flaps 1.
Post-accident flight tests conducted in a Boeing
737-300 aircraft showed conclusively that 190 KIAS was at or very near the
"crossover speed" for the weight and configuration of USAir Flight 427. At
this speed, ailerons and spoilers were sometimes insufficient to stop the
roll induced by a full rudder deflection. Charts Boeing provided to the NTSB
in June 1997 suggest that B-737 crossover speeds increase with bank angle
(actually angle of attack or G loading).
Initial investigation efforts in the Boeing MCAB
simulator indicated the aircraft could be recovered from a hardover rudder
at 190 KIAS/Flaps 1. However, this testing occurred before the simulator
was modified to reflect the aircraft's actual rudder authority. Similarly,
the FAA Critical Design Review team found recovery from a full rudder hardover
at 190 KIAS/Flaps 1 to be very difficult in the MCAB simulator. These tests
also occurred before the simulator had been modified to reflect actual B-737
rudder authority. >From the CDR team's description of the recovery attempts,
it is clear that more rudder authority would have made a successful recovery
After the Boeing MCAB simulator was modified
to reflect the actual rudder effectiveness found in the aircraft, the FAA's
Mr. Berven, one of the pilots who flew the test flights, experimented with
sudden hardover rudder deflections in the simulator. He concluded that if
a B-737-300 aircraft cruising at 190 knots with Flaps 1 encountered a hardover
rudder, recovery was impossible if the pilot attempted to maintain altitude.
Recovery under those conditions was possible only if the pilot descended
to gain airspeed, which decreases rudder effectiveness and increases
aileron/spoiler authority enough to overcome the roll. However, he also stated
that airline flight crews were unlikely to take such action, as their natural
reaction would be to maintain altitude, particularly while analyzing a control
The manufacturer's pilot handbook for the B-737
did not contain a procedure for recovering from a hardover rudder. Moreover,
the airline industry was not aware that the manufacturer's recommended
maneuvering speed for USAir Flight 427's configuration and weight placed
the aircraft at or very near the speed at which full lateral controls were
insufficient to stop the roll induced by a dynamic hardover rudder or rudder
reversal. Indeed, at the time
of the USAir 427 accident, the manufacturer had
not informed the airline industry of the "crossover speed" concept.While
unknowingly cruising at or near the crossover speed for their weight and
configuration, the crew of USAir Flight 427 encountered the wake vortex of
a preceding aircraft. The Flight Data Recorder from USAir Plight 427 indicates
a full-left rudder movement occurred approximately four seconds after the
wake vortex encounter began. At that time, the aircraft was in approximately
18° left bank and maintaining level flight. This uncommanded, dynamic,
full-left rudder movement occurred when the aircraft was at or below the
"crossover speed" and an uncontrollable yaw and roll to the left resulted.
The crew's training and piloting sense dictated that they should maintain
altitude while analyzing and correcting the problem, in order to preserve
maneuvering room and available time in which to effect a recovery. At the
onset of the rudder movement, the crew took reasonable action to counteract
the roll with lateral controls while attempting to maintain altitude as they
dealt with the situation. Unknown to the crew, these actions quickly placed
the aircraft in a position from which recovery was impossible.
b. Unusual Attitude Training
i. Introduction US Airways believes unusual
attitude training is useful and appropriate for airline flight crews. US
Airways has always incorporated unusual attitude maneuvers, concepts and
techniques into its pilot training programs. In addition, all USAir pilots
received unusual attitude recognition and recovery a number of times prior
to becoming airline pilots. In certain circumstances, unusual attitude
recognition and recovery techniques can be critical to the safety of flight.
However, no amount of unusual attitude training could have prevented this
The full-left rudder deflection combined with
the then-unknown crossover speed of the B-737 aircraft prevented the recovery
of Flight 427 notwithstanding the flight crew's application of proper recovery
ii. Factual Investigation The comments
made by the pilots indicate they were fully aware of the aircraft's attitude,
but were unable to change it. Cohen Report at 2. Captain Cox thought unusual
attitude training would not be pertinent if the unusual attitude resulted
from a deflected flight control combined with a lack of full authority over
all three axes of flight. Cox Tr. at 2177.Additional training in recovery
from high bank angle, nose low attitudes has been considered and implemented
by several operators, including US Airways. American Airlines pilots are
taught that when bank angle is less than 60° in a nose low unusual attitude,
the pilot should increase back pressure on the yoke. February 28, 1996 letter
from Curt Graeber and Mike Carriker to Malcolm Brenner, at Part II, No. 17.
Flight Safety International recommends that during a nose low recovery the
pilot should increase back pressure immediately if the bank angle is less
than 90°. Id. at 28. US Airways' Selected Events Training program teaches
pilots to apply back pressure in a high bank, nose low recovery after the
bank is reduced to less than 60°.
iii. Analysis All unusual attitude training
assumes proper functioning of flight controls. Here, the rudder moved to
an uncommanded full deflection at an airspeed which precluded recovery by
use of lateral controls. Unusual attitude training was, therefore, completely
irrelevant because the aircraft was not responding in a way that allowed
recovery by application of unusual attitude recovery techniques.In any event,
the actions of the crew of USAir Flight 427 conformed with unusual attitude
recovery procedures, including those published after the accident. As the
uncorrunanded nodder movement began, the aircraft yawed and rolled to the
left and the nose began to drop. With a nose low, left bank attitude, the
proper procedure is to counter the roll with opposite aileron/spoiler and
increase back pressure on the yoke to reduce altitude loss, unless the bank
angle exceeds 60-90 degrees. This is exactly what the USAir 427 flight crew
did. As the bank angle continued to increase, and the nose continued to drop,
the flight crew disconnected the autopilot, added additional right
aileron/spoiler, and increased back pressure on yoke. This, too, is consistent
with proper procedure. The aircraft reached 60° of bank just over 5
seconds after the uncommanded rudder movement began, approximately 1 1/2
seconds before the aircraft stalled. At 60° of bank, the flight crew
had applied approximately 2/3 of the available aft yoke authority and full-right
aileron/spoiler, again as appropriate under the circumstances. The aircraft
stalled at approximately 70° of bank, approximately seven seconds after
the full-left rudder movement began.
In hindsight, it can be said that the proper
procedure would have been to increase airspeed by descending until the aircraft
accelerated through the crossover speed. This is not, however, an unusual
attitude recovery technique. It is a technique to recover from a B-737 hardover
rudder or rudder reversal. It is also a technique that is at odds with unusual
attitude recovery procedures and natural piloting instincts.
Because proper unusual attitude recognition and
recovery techniques were followed, and previously unknown factors were at
worl: on the aircraft, additional unusual attitude training would not have
prevented this accident.Under any circumstances then known to the airline
industry, the actions of the crew of USAir Flight 427 were reasonable and
correct. Unfortunately, the crew encountered an unconunanded full rudder
deflection at or below the crossover speed and entered a flight regime wholly
unknown at the time to this crew or the airline industry. Under these
extraordinary circumstances, the crew's application of the correct flight
control inputs in an attempt to recover from an uncornmanded yaw/roll/descent
quickly placed the aircraft in an unrecoverable situation.The actions of
this crew cannot be judged with the benefit of 20/20 hindsight which is itself
based on over three years of intense investigation and analysis. This crew
had seven seconds, at most, in which to recognize, analyze, and recover from
a previously-unknown malfunction. Worse yet, this crew faced the malfunction
in a flight regime where recovery was impossible unless the crew reacted
in a way that was contrary to their training and natural piloting instincts.
1. The Captain and First Officer were trained,
certificated and qualified for the flight in accordance with applicable
2. Nothing in the flight crew's background suggests
they would have had problems with disorientation or control of the accident
3. The flight crew's performance was not affected
by illness, fatigue, or personal or professional problems.
4. The aircraft was properly maintained in accordance
with applicable regulations. Inspections of the rudder control system required
by AD 94-01-07 had been correctly accomplished in a timely manner.
5. It was daylight and the weather was clear
with a distinct horizon at the time of the accident.
6. The accident aircraft's speed and configuration
at the beginning of the accident event complied with the manufacturer's and
operator's maneuvering speed schedules.
7. At the beginning of the accident event, the
aircraft was at or below the "crossover speed," which is the speed below
which lateral flight control authority is insufficient to counter the roll
induced by a full rudder deflection.
8. The manufacturer did not advise the operator,
prior to this accident, that there were speeds below which B-737 lateral
flight control authority is insufficient to counter the roll induced by a
full rudder deflection.
9. The manufacturer's published maneuvering speeds
for some weights and configurations of the Boeing 737 were too slow and did
not provide sufficient airspeed margins to allow recovery from an uncornmanded,
fully deflected rudder or rudder reversal.
10. The accident aircraft's rudder moved uncommanded
or reversed to the full-left position.
11. At the onset of the full rudder movement,
the accident aircraft's speed was at or below the "crossover" speed.
12. The manufacturer did not provide the operator,
prior to this accident, with an emergency procedure for recovery of a Boeing
737 from an uncornmanded, full rudder deflection or rudder reversal.
13. Based on information known to them at the
time, the flight crew reacted correctly to the uncomrnanded, full rudder
deflection or rudder reversal and resultant left roll by selecting opposite
aileron and attempting to maintain altitude.
14. After the onset of the full rudder movement,
decreasing airspeed, increasing bank angle, and increasing aerodynamic loads
kept the aircraft's speed below the "crossover" speed.
15.With an uncommanded, fully-deflected rudder
or rudder reversal and the aircraft below the "crossover" speed, recovery
through techniques known at the time was not possible.
The probable cause of this accident was an
uncommanded, full rudder deflection or rudder reversal that placed the aircraft
in a flight regime from which recovery was not possible using known recovery
A contributing cause of this accident was the
manufacturer's failure to advise operators that there was a speed below which
the aircraft's lateral control authority was insufficient to counteract a
full rudder deflection.